This project deals with the mechanism of prostaglandin biosynthesis using the bovine seminal vesicle microsomal system. (Plus or minus)-Eicosa-cis-14,15-epoxy-cis-8,11-dienoic acid was synthesized and was found not to be a prostaglandin biosynthetic intermediate. Instead, it was converted into eicosa-14,15-dihydroxy-cis-i-11-dienoic acid by the prostaglandin synthesizing system. Eicosa-13-2H2-cis-i,11,14-trienoic acid was synthesized to delineate the rate-determining steps in prostaglandin biosynthesis by comparing its rate of oxidative cyclization with that of non-deuterated eicosa-cis-8,11,14-trienoic acid. It was found that the hydrogen-abstraction step is not the rate-determining step, even though it contributes to the rate of the overall reaction.